1. Field of the Invention
This invention relates to thermoplastics and more particularly relates to elastomeric polyurethanes and medical articles therefrom.
2. Background of the Invention
Polyurethanes possess an outstanding balance of physical and mechanical properties and superior blood compatibility compared to other polymers such as silicone rubber, polyethylene, polyvinyl chloride and perfluorinated polymers. As a result, they have come to the fore as the preferred polymeric biomaterials for fabrication of various medical device components. Some important device applications for polyurethanes include peripheral and central venous catheters, coatings for heart pacemaker leads and the Jarvik heart.
As known in the art, polyurethanes are synthesized from three basic components, a diisocyanate, a polyglycol and an extender, usually a low molecular weight diol, diamine, or water. If the extender is a diol, the polyurethane consists entirely of urethane linkages. If the extender is water or diamine, both urethane and urea linkages are present and the polyurethane is termed a polyurethaneurea.
Polyurethanes develop microdomains conventionally termed hard segments and soft segments, and as a result are often referred to as segmented polyurethanes. The hard segments form by localization of the portions of the polymer molecules which include the isocyanate and extender components and are generally of high crystallinity. The soft segments form from the polyglycol portions of the polymer chains and generally are either noncrystalline or of low crystallinity. One of the factors which determines the properties of the polymer is the ratio of hard and soft segments. In general, the hard segment contributes to hardness, tensile strength, impact resistance, stiffness and modulus while the soft segment contributes to water absorption, elongation, elasticity and softness.
Polyurethanes chain extended with diols have been extensively studied for biomedical application. Exemplary of important diol extended polyurethanes are: VIALON.TM. (Becton, Dickinson and Company) PELLETHANE.TM. (Upjohn Chemical Co.,) and TECOFLEX.TM. (Thermedics Inc.) These proprietary products typically have good blood compatibility, but, with the exception of VIALON.TM., generally require processing additives such as antioxidants and detackifiers, a potential disadvantage for use in biomedical articles. They are, however, thermoplastic and therefore may be extruded and injection molded.
U.S. Pat. No. 4,202,957 to Bonk et al. discloses a polyurethane from a diisocyanate, an extender and a particular polyoxyethylene-polyoxypro pylene glycol which is stable up to 450.degree. F. and therefore is melt processable without decomposition. In U.S. Pat. No. 4,822,827, Bonk et al. further improve heat stability for thermoplastic processing by including a cycloalkane diol extender in the formulation.
A softening and swelling catheter fabricated of a polyurethane synthesized from polyethyleneoxide soft segment is disclosed in U.S. Pat. No. 5,061,254 to Karakelle et al. of common assignee herewith.
A multilumen catheter marketed under the trade name FLEXTIP.TM. by Arrow International Corp., Wilmington, Del. and disclosed by Howes in U.S. Pat. No. Re. 31,873 and by Botterbusch et al. in U.S. Pat. No. 5,004,456 consists of a relatively soft distal end segment intended for insertion into a body cavity and a relatively hard rigid portion joined thereto by heat and/or pressure. The catheter segments are polyurethanes from aliphatic or aromatic diisocyanates.
While significant improvement in catheter performance has resulted from the above disclosures, there remains a need for a radiation stable polyurethane having the blood compatibility necessary for catheter manufacture which is stiff when dry for catheter insertion but which becomes soft and pliable for positioning and indwelling. The present invention addresses this need.